Post‐drought community turnover and functional redundancy in a tropical forest understorey

Questions Drought events are increasingly frequent, threatening the biodiversity of tropical forests. The understorey comprises a large fraction of the total plant species richness of these systems with the presence of highly diverse angiosperm families. Here, we quantified the effects of a drought on abundance and functional structures and on the ecosystem functioning of Rubiaceae assemblages along a topographic gradient. Location Mata das Flores State Park, an Atlantic Forest fragment in southeast Brazil. Methods Two vegetation surveys were performed: one during an El Niño‐induced drought, and the other three years after this drought. Abundance and functional structures were assessed using the 16 most‐abundant species of Rubiaceae, which comprised 92% of the total abundance. A litter decomposition experiment was carried out to estimate the percentage of mass loss by the Rubiaceae species. We carried out Procrustes analyses on abundance and functional structures and used Generalized Linear Mixed Models to test the effects of drought and topographic habitats on taxonomic and functional compositions, and mass loss. Results We found that the functional structure and mass loss remained constant despite significant changes in the abundance structure after the drought. Conclusion The evidence points to the maintenance of ecosystem functioning through functional redundancy, because functionally similar less‐abundant species replaced each other after the drought. We show that swarms of species can maintain biological diversity and stability in ecosystem functioning under drought in the understorey of a tropical forest. Drought events are becoming more frequent, posing a threat to the biodiversity of tropical forests. We quantified the effects of drought on species abundance, functional structure and ecosystem functioning of Rubiaceae assemblages along a topographic gradient. Our findings demonstrate that despite significant changes in species abundance following the drought, both functional structure and ecosystem functioning remained constant.